One final note. The value A27, the stat weight for armor, is fairly worthless information because it doesn't take into account any armor multipliers you have from talents and your tanking stance. This doesn't really matter though as you get armor naturally, and you can't trade armor for any of the other tanking stats anyway.
I have to quibble a little with this point, because I, and probably many others, don't have any Underworld set armorings, whereas we can buy Verpine armorings. Actually, when I first hit 55, I only had Campaign armorings, and as Kitru and others have said elsewhere, the benefits of the set bonus outweighs the benefit of the 69/72 armorings. Even now, having the 4-piece Arkanian bonus, that's still 4 Verpine armorings I can't use.